This invention relates to plastic injection molding and, more particularly, to precision molding methods and apparatus for forming miniature optical components for use in optical fiber connectors, for example.
The effectiveness of optical fibers for transmitting information by light is now well known and the basis for increased commercial application of fiber optics systems. As such systems evolve, however, the need for improved, low-cost fittings, such as connectors for coupling optical fiber segments, becomes increasingly apparent. In spite of the apparent need, the extremely small diameters of the fiber cores used (on the order of 50 microns or 0.002 inches) and propensity for light loss at any discontinuity in the refractive index of the light path have severely curtailed the attainment of a low-cost optical fiber connector which is capable of performance in a manner consistent with the light transmitting efficiency of a continuous optical fiber.
Fiber optics connectors heretofore disclosed have generally involved precision molding or otherwise forming an enlarged plastic or metal terminal component at or near the end of each of two fibers to be connected, taking great care to achieve seating surfaces on the terminal components which are concentric with the fiber axis. Two such terminals may then be joined mechanically to retain the connected fibers in precise alignment with each other. While such connectors may be considered releasable or reconnectable in the manner of an electrical coupling, the need for a liquid or plastic having the same refractive index as the fibers to effect continuity of light transmission through the connection has tended to a class of connectors more truly characterized as a quasi-permanent splice than a releasable connector. Exemplary disclosures of such connectors are found in U.S. Pat. Nos. 3,999,841; 4,087,158; 4,107,242; and 4,173,389.
To avoid the problems associated with a connector design in which the two optical fibers are, in effect, retained in end-to-end abutment for direct transfer of light from one fiber to the other, it has been proposed to use a preformed fiber terminal mountable over the end of each fiber in a manner to be self-centering and incorporating a collimating lens by which a beam emerging from one fiber end is enlarged and refocused into the outer fiber end. Thus, arranging the lens of one such terminal in facing axial relationship to the lens of another such terminal effects a transfer of information from one fiber to the other by way of an enlarged collimated beam which is subsequently reduced for transfer to the second optical fiber. The preformed terminal is not only easily applied concentrically to the end of each fiber, but the transfer by way of an enlarged beam contributes to higher light-transmitting efficiencies due to the greater range of dimensional tolerances permitted in the mechanical components for releasably retaining the two terminals in operative relationship. Additionally, the conversion of the fiber optic transmitted light signals to an enlarged beam at the juncture of the two fibers permits use of beamsplitters and the like for monitoring or otherwise tapping the information represented by the transmitted light.
Although in theory, this latter class of fiber optic connectors represents an exciting advance in the field of fiber optics, practical application has been disappointing largely due to an inability to meet required mechanical and optical tolerances. In a molded plastic terminal component having overall exterior dimensions of less than 1/4 inch in diameter and approximately 3/8 inch in length, for example, a seating surface for an optical fiber having a composite outside diameter of approximately 0.005 inches must be concentric on the axis of an aspheric lens within 0.000020 inches; the lens must be focused precisely at the end of the seated optical fiber and the lens must be capable of alignment with a similar lens to 0.5 minutes of arc. The maintaining of such tolerances by injection molding of plastic materials presents a major challenge which heretofore has not been met.